It is known to manufacture methyl esters using conventional means such as homogeneous catalysis with soluble catalysts, such as soda or sodium methylate, by reacting a neutral oil and an alcohol such as methanol (for example JAOCS 61, 343-348 (1984)). A pure product that can be used as fuel and a glycerin meeting the specifications are however obtained only after many stages. In fact, the glycerin obtained is polluted by alkaline salts or alcoholates, so that the glycerin purification plant is almost as costly as the ester manufacturing plant.
Heterogeneous catalysis methods afford the advantage of producing catalyst-free esters and glycerin, which are therefore easily purified. However, it is often difficult to economically obtain both an ester and a glycerin of high purity.
European patent EP-B-0,198,243 describes the manufacture of methyl ester by transesterification of an oil with methanol, using as the catalyst an alumina or a mixture of alumina and of ferrous oxide. However, the liquid hourly space velocity (volume of oil injected/volume of catalyst/hour) is low, the amount of glycerin collected is much less than that theoretically expected and the purity of the esters obtained is rather low (ranging between 93.5% and 98%).
Methods using a catalytic system based on metallic oxides, alone or in combination, deposited or not on an alumina, have been described. Patent FR-B-2,752,242 filed by the applicant describes the use of solid and non soluble catalysts formed from alumina and zinc oxide or zinc aluminate. Patent applications EP-A-1,505,048 and EP-A-1,593,732, also filed by the assignee, describe a method of transesterification of vegetable or animal oils by means of heterogeneous catalysts based on mixtures of alumina and titanium oxides, alumina and zirconium oxide, alumina and antimony oxide or combinations of zinc and titanium oxides, alumina, zinc and titanium oxides, bismuth and titanium oxides or alumina, bismuth and titanium oxides.
Besides these oxide type solids, an increasing number of new basic phases has been used to catalyse the transesterification of oils with alcohols.
By way of example, De Filippis et al. (Energy & Fuels, 2005, 19, 225-228) suggest the use of sodium phosphate for catalysing the rapeseed oil transesterification reaction. Although the authors point out that this type of solid can be reused in two successive cycles with a 1% activity loss, nothing shows that the resistance of the materials is sufficient over long periods of time. In fact, some problems due to the presence of water in the feed are mentioned, notably swelling of these phases and wall adhesion.
Suppes et al. (Applied Catalysis A: general 257 (2004) 213-223) use different materials such as Cs or K-exchanged zeolites or metals that go into the composition of the reactors for transesterification of soybean oil.